docs: token ergonomics -- runtime routing/budgets/caches and the development-side tooling

One page, two halves with a Mermaid diagram each. A: the plane's token economy (one Model per Task with a metered usd/calls budget, attribution table, credentials only in the start request, repo-facts and content-addressed reuse, replay at $0, triage-hunt-tie-break order, prefix-cache prompt order) with every cost quoted from plane-increment-1/2.json, the injection-slice and harvest records, and an honest not-yet list (no token budget in ax, triage applied not run, k and contrast fixed outside experiments). B: the development-side measurement (tool I/O 85.5%, previously 90%) and the benchmarks/dev remedies: read_guard hook, bulk_read portal, token_report, plus the practices. Linked from the nav, index, memory-and-detection and the README docs list.

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

docs/index.md

@@ -49,6 +49,8 @@ flowchart LR
 - Understand a finding's evidence and score: [Architecture](architecture.md).
 - Operate the plane and its store: [ax on this host](ops/ax/README.md) and
   [RustFS setup](rustfs.md).
+- Where the model spend goes, per Task at runtime and in the maintainers' own coding sessions,
+  and what is reused instead of paid again: [Token ergonomics](token-ergonomics.md).
 - Trust boundaries and hardening: [Threat model](threat-model.md).
 
 Every number in these pages is quoted from a committed record whose path is given next to it.